This invention relates to methods for manufacturing heat exchangers such as radiators or condensers for motor vehicles and the like and, more particularly, to an improved process for brazing components of an aluminum heat exchanger.
There are conventional evaporators, condensers, radiators and the like, such as for motor vehicles, which consist of an aluminum heat exchanger manufactured by brazing. For such manufacture, the components of the heat exchanger are assembled together with brazing material, and the assembly is then heated in a furnace so that the components are brazed together. Each of the heat exchanger components includes an inner portion made of an aluminum alloy consisting of pure aluminum and a metal such as manganese and a coating portion containing brazing material cladded on the surface of the inner portion. The brazing material in the coating portion is an aluminum alloy which contains an element such as silicon and which has a lower melting point than the aluminum-manganese alloy. The brazing material is coated on that part of the inner portion which is to be brazed. The components consisting of the inner aluminum alloy portions and the outer coating portions are then assembled. Alternatively, the components of the heat exchanger may consist of the inner portions and layers of the brazing material which are simply placed on the parts of the inner portion which are to be brazed. After the components have been assembled, a flux is applied to those parts of the inner portions of the members to be brazed, and the components are then brazed in a furnace so that they are joined together. If the components are to be brazed to each other in a vacuum furnace, a flux need not be applied to them.
FIG. 5 of the drawings shows a conventional parallel flow aluminum heat exchanger 100 of the type disclosed in Japanese Utility Model Application Laid-open No. 28,980/90 and Japanese patent application Laid-open No. 207,57287. In this case, the heat exchanger 100 include a core 104, a pair of spaced header tanks 101 made of aluminum, and some reinforcing plates 105. The core 104 includes a plurality of flat tubes 102 extending between the header tanks 101 so as to carry a fluid which is subjected to heat exchange by the heat exchanger, and a series of corrugated fins 103 mounted between the tubes to act as heat transfer members. The core 104 is disposed between the header tanks 101 so that the flat tubes 102 communicate with the tanks at both ends. The flat tubes 102, the corrugated fins 103, the header tanks 101, and the reinforcing plates 105 are brazed together in a furnace so as to form the heat exchanger 100.
Each of the header tanks 101 is formed from a pipe which has a circular cross-section, originally open at both ends, and has an inner portion 101a made of an aluminum alloy and a brazing material 101b coated on the peripheral surface of the inner portion so that the combination constitutes the pipe as shown in FIG. 6. At each end, the header tank 101 also includes a lid 106 made of an aluminum alloy and provided with a brazing material on the outside surface to braze it to the pipe so as to close the pipe at both ends. The pipe also has a series of flat tube insertion holes 107 into which the ends of the flat tubes 102 are inserted. The corrugated fins 103, which are made of an aluminum alloy and are not coated with brazing material, are inserted between the flat tubes 102. The pipes of the header tanks 101 have inlet and outlet tube-receiving holes in which inlet and outlet pipes 108 and 109 made of an aluminum alloy are inserted The reinforcing plates 105 are cladded with a brazing material on the sides which face the core 104.
As shown in FIG. 7, which is a cross-sectional view of a tube 102, each of the flat tubes 102 includes an inner portion 102a made of an aluminum alloy and a brazing material 102b cladded on the outer surface of the inner portion. An inner fin 110 is inserted into each of the flat tubes 102 so that the internal space R thereof is divided into a plurality of fluid passages r. The inner fin 110 functions to improve the transfer of heat from the fluid to the tube 102 and to enhance the strength of the tube against lateral pressure. The inner fin 110 includes a thin inner portion 110 made of an aluminum alloy and a brazing material 110b coated on both sides of the inner portion.
It is well known that, when a flat tube such as the flat tube 102 is manufactured, a band of sheet material is bent by forming rollers so as to butt both of the side edges of the sheet against each other, the side edges are then seam-welded to each other, and the bent and welded sheet is thereafter cut off to a prescribed length. This manufacturing process is illustrated in FIG. 8. As shown therein, the band of sheet material, which is flat and long, is bent at the centerline of the sheet to a U shape by forming rollers so that the sheet is gradually formed into a flat tube to butt the side edges 102c and 102d of the sheet against each other. Electric current is thereafter applied in an argon gas atmosphere to the abutting side edges 102c and 102d through a roller electrode 115 which engages the edges and is connected to a rotary welding transformer 114, and the sheet formed as the flat tube is pressed on both sides and at the bottom, so that the edges are welded to each other. This produces the flat tube 102.
When the heat exchanger 100 is manufactured, the surfaces of the inner fins 110 are coated with a noncorrosive flux and the fins are inserted into the tubes 102. The corrugated sheets 103 are inserted between the flat tubes to form the core 104 of the exchanger, and the ends of the tubes are inserted into the flat tube insertion holes 107 of the header tanks 101. In addition, the reinforcing plates 105 are mounted on the top and bottom of the body. All of these components are held together with jigs, a noncorrosive flux is applied to the brazed parts of the components in the usual manner, and the components are then brazed to each other in a furnace so that the flat tubes are joined to the inner fins and the corrugated fins and to the header tanks at the flat tube insertion holes thereof. Since the flat tubes 102 and the corrugated fins 103 are stacked alternately and the assembly is compressed so as to produce the spacing between the tubes, there is a relatively large variation in that spacing. Consequently, if the clearance between the ends of the flat tubes 102 and the header tank 101 at the flat tube insertion holes 107 thereof is not large enough, it is likely that one or more of the tubes cannot be inserted into its insertion hole. To overcome this problem, the clearance between the tube ends and the holes is increased However, if this clearance is increased too much, there will be insufficient brazing material 102b on the flat tube 102 to braze the tube securely to the header tank 101 at the tube insertion hole 107. As a result, the clearance will not be filled completely with brazing material, leaving a hole which will permit fluid to leak out of the heat exchanger. To deal with this problem, brazing material is provided on the header tanks 101 at the flat tube insertion holes 107 thereof by cladding of the tanks with the material or by placing it on the tanks prior to brazing.
Since butt-seam welding is performed in the above-described method of manufacturing the flat tube 102, the manufacturing efficiency is not good and expensive welding equipment is required.
To overcome this problem, a flat tube 116, shown in FIG. 9, is brazed to a header tank with which the tube is in surface contact. The flat tube 116 is of the type disclosed in Japanese published Application No. 35,830/91 and has side edge projections 116d extending from one side of the tube, as shown in FIG. 9. To manufacture the flat tube 116, a band of sheet material including an inner portion 116a and a brazing material 116b coated or cladded on the outer surface of the inner portion is folded and formed. The brazing material 116b is used to braze the tube 116 to corrugated fins 103 on opposite sides of the tube, as shown in FIG. 10, and to the header tanks and to fill the clearance between the tube ends and the tanks at the flat tube insertion holes therein. The tube 116 also includes another coating of brazing material 116c on the inner surface of the inner portion 116 to braze the engaging surfaces 116e of the side edge projections 116d to each other and to braze the tube to inner fins 117 which are inserted in the tube as shown in FIG. 10. Initially, the height h.sub.2 of the space within the flat tube 116 for the inner fin 117 is slightly greater than the height h.sub.1 of the inner sheet, and there is a gap S between the side edge projections 116d, as shown in FIG. 9. After a noncorrosive flux is applied to the inner fin 117, the fin is inserted into the flat tube 116 and the tube is then deformed slightly by pressing, so that the tube has a predetermined height H, as shown in FIG. 10.
Thereafter, all of the heat exchanger components including the flat tubes 116 are held together with jigs, a noncorrosive flux is applied to the coated parts of the components, and they are then heated in a furnace in the manner described above. As a result, the engaging surfaces 116e of the side edge projections 116d of the flat tubes 116 are brazed to each other, the tubes are brazed to the corrugated fins 103 and to the header tanks, and the inner fins 117 are brazed to the inner surfaces of the tubes.
However, it has been found that a minute hole is generated in the flat tube 116 near the end received in the header tank opening at the time of heating. It is believed that the minute hole is produced because the temperature of the flat tube 116 and the corrugated fin 103 rises more quickly than that of the header tank during heating in the furnace because the fin materials used for the tube and for the corrugated fin are thinner than the header tank material in order to enhance the heat-exchanging property of those components.
This phenomenon is described as follows. Since the temperature of the flat tube 116 and the corrugated fin 103 rises more quickly than that of the header tank, the brazing materials 116b and 116c of the tube melt sooner than the brazing material on the tank. When the brazing material on the tank does melt, there is a clearance between the tube 116 and the tank at the flat tube insertion hole. Consequently, the molten brazing material from the tank flows into that clearance and comes into contact with the ends of the engaging surfaces 116e of the side edge projections 116d of the tube so that much of the molten material flows between the engaging surfaces due to capillary attraction. As a result, the silicon from not only the brazing material 116c of the flat tube 116 but also from that of the brazing material of the header tank diffuses into the inner portion 116a of the tube at the engaging surfaces 116e so that the aluminum alloy of the inner portion at the engaging surfaces changes into an aluminum-silicon alloy having an excessive silicon content which is lower in melting point than the aluminum alloy. For that reason, the inner portion 116a is eroded by both the brazing materials so that a minute hole is generated in the flat tube 116. It is difficult to find the minute hole by the naked eye, and the hole usually cannot be found until the inspection after completion of the heat exchanger. This reduces the production yield of heat exchangers in the manufacturing process.